The present invention generally relates to precision approach radar (PAR) systems for displaying information to an air traffic controller useful for guiding an aircraft along a preferred glideslope approach to a runway.
Precision approach radar systems for guiding an aircraft to a runway touchdown point are well known. Such systems typically use a pair of radar beams in a microwave radio frequency (RF) band emitted from a radar head disposed proximate to the runway touchdown point. The radar beams are respectively scanned horizontally and vertically to determine the relative azimuth and elevation positions of one or more targets, e.g., aircraft, in the vicinity of the radar head. In an early exemplary system, as disclosed in U.S. Pat. No. 2,649,581 to Tasker et al., a single monitor, e.g., a CRT (cathode ray tube) video monitor, displays side and top views of a preferred glideslope and the relative position of any detected aircraft. The display is generated by sweeping electron beams across the face of a CRT that are synchronized to the scanning of the radar beams from the radar head. These swept electron beams are typically highlighted in response to video signals produced by analog receivers in response to the radar beams reflected from targets. A typical sweep pattern repeats at least once a second and phosphors on the CRT are chosen so that patterns persist long enough for viewing by an operator, albeit in a somewhat darkened room. The video signal resulting from the reflected radar beam is typically subject to various noise sources from ground obstructions (e.g., mountains or towers), weather systems, as well as various sensitivities to different portions of each target. Filters are typically used to minimize these noise sensitivities to try to determine the "true" relative position of the aircraft. While such systems are relatively effective, they impose a significant workload on highly-trained operators to interpret the resultant noisy and flickering (due to the persistence of the CRT phosphors) display. The operator is presented with a bright line of video, indicating the scanning of the radar, and has to detect targets in the dim residual persistence trails on the screen. This leads to operator visual fatigue. A paper by E. H. Reitan and S. H. Saib presented at the Institution of Electrical Engineers (IEE) International Conference on "The Future of Aircraft All-Weather Operations," London, England, Nov. 22-29, 1976 entitled "Interactive Computer Graphics in an All-Weather Landing System" described an earlier implementation of a synthetic color random scan display system to reduce the operator's workload.